Optical disk drives capable of data recording and playback using optical media such as CDs and DVDs have become widespread in recent years. A photoreceptor IC provided in the optical pickup within such an optical disk drive has a function for converting reflected laser light from a disk to current using a light-receiving circuit and converting the current to voltage using a current-voltage (I-V) converting circuit. Since the intensity of light that is input to the light-receiving element differs depending upon the operating conditions and there is a difference in laser output intensity conforming to the playback operation and a difference in the reflectivity of each optical disk, there are cases where the intensity becomes several times the saturation level of the output in the current-voltage converting circuit. To deal with this problem, Patent Document 1 discloses a current-voltage converting circuit for limiting input current in such a manner that excessive current will not be input to the current-voltage converting circuit.
FIG. 5 is a circuit diagram illustrating a current-voltage converting circuit described in Patent Document 1. The illustrated circuit arrangement is such that when photocurrent of a photodiode (light-receiving element) 102 becomes a large current that exceeds the output saturation level of a current-voltage converting circuit 101, the input current is limited in order to prevent an abnormal output response waveform that accompanies saturation of a transistor within the current-voltage converting circuit 101. More specifically, a constant-current circuit 111, current mirror circuits 109 and 110, and an NPN transistor 108 for dealing with superfluous current are provided between the light-receiving element 102 and the current-voltage converting circuit 101.
If an optical current ascribable to the light-receiving element 102 does not exist, the current values of the current mirror circuits 109 and 110 driven by the constant-current circuit 111 are equal and therefore the input current to the current-voltage converting circuit 101 is zero. On the other hand, if an optical current ascribable to the light-receiving element 102 is produced, this optical current flows into the current mirror circuit 109. As a result, the current of the current mirror circuit 109 increases in comparison with the current mirror circuit 110 and the difference current becomes the input to the current-voltage converting circuit 101.
As a result of the foregoing, a voltage drop across a resistor 104 is decided by the value of the current that flows through the current mirror circuit 109, namely by the sum of the current value decided by the constant-current circuit 111 and current mirror circuit 110 and the optical current. If the voltage drop across the resistor 104 increases owing to an increase in the optical current, the NPN transistor 108 is turned on by a certain optical current value, the optical current is diverted and the inflow of current to the current mirror circuit 109 is limited. By virtue of this operation, the input current to the current-voltage converting circuit 101 is limited to a value less than the optical current value at which the NPN transistor 108 turns on attendant upon the voltage drop across the resistor 104.
[Patent Document 1]
    Japanese Patent Kokai Publication No. JP-P2006-101110A